<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>@container</title>
    <style>
      .day {
        container-type: inline-size;
        container-name: calendar-day;
      }

      .date {
        display: inline-block;
      }

      .date-num {
        font-size: 16px;
        display: inline-block;
      }

      @container calendar-day (max-width: 200px) {
        .date {
          display: block;
        }

        .date-num {
          font-size: 2.5rem;
          display: block;
        }
      }
    </style>
  </head>
  <body>
    <div class="day">
      <div class="date">
        <p class="date-num">1212</p>
      </div>
      <div class="date">empty</div>
    </div>
  </body>
</html>
